2004 YZ450F Losing 40-50% of Compression

Hi all, I recently purchased a non running 04 YZ450F, the guy who I got it from couldn't tell me much about it. When looking it over I found it had no compression and wouldn't fire.


I did some searching on here and someone recommended listening for air hissing when kicking it, sure enough I could hear air running back through the carb when kicking it over. I took it to Yamaha and had them shim the valves and verify top dead center on the timing.


When I picked the bike up they told me it was still not running and was still losing 40-50% of it's compression. They suggested it's something wrong with the head itself and most likely not the piston or rings. It kicks over fine, though it is probably too easy (this is my first 4 stroke). They wanted $1500 to tear the engine down, I'm fairly confident I can get it apart and back together, but have no idea what to look for or where to start.


How does the auto decompression work on these bikes, could it be causing my compression to escape?


How about the hot start lever?


Is it more likely a valve not seating in the head, and would a complete head off ebay remedy this?




Thank you very much in advance for any advice,

Don't get ahead of yourself.


The first thing to do would be to have a leak down test done at a local shop.  This will tell you what's leaking compression and how much. 


Auto decompression is not likely to be the cause of any problems. Read: http://www.thumpertalk.com/topic/593945-06-yz450f-compression-issue/#entry5732518


Once you know where the problem is, you can go from there.

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